Image sensors which are used in a spectral range from hard UV, through x-rays to gamma-rays, i.e. with photon energies of 10 eV or greater, or in the imaging of energetic particles, e.g. free electrons, almost invariably produce a yield of many signal electrons per incident energetic photon or particle. Such high yields may be undesirable, particularly for small pixels, e.g. of 40 μm or less, designed for the visible spectral region, where only one signal electron is generated per incident photon. A common result when imaging energetic photons, is premature saturation of the pixel, and a low signal to noise ratio, limited by photon shot noise.
Such image sensors may include a sensitive layer coupled to a charge collection/readout structure, e.g. photoconductor or scintillator on a CMOS array, or may include an intrinsically sensitive charge collection/readout structure, e.g. deep active layer CMOS.
In a scintillator-based imager, the imager is coated with, or attached to, a scintillator layer. The scintillator converts, for example, incident x-ray photons to optical photons, and the image sensor converts the optical photons to an electrical signal. X-ray sensors based on a scintillator and a photodetector, typically a CMOS sensor, which converts the optical photons to an electrical signal, are well known. Intra-oral dental x-ray imaging typically uses such sensors.
An optimum x-ray sensor requires a satisfactory balance between x-ray absorption in the scintillator and spatial resolution for a particular scintillator. A thin scintillator provides low light levels to the detector, but has a high spatial resolution. A thicker scintillator stops more x-ray photons, and provides higher light levels to the detector, up to a thickness at which absorption and scattering leads to fewer light photons being transmitted through the scintillator to the detector, but has a lower spatial resolution because of scattering in the scintillator. Even with a fixed thickness of a same scintillator material, obtaining a constant light level per unit x-ray dose to the sensor may not be straightforward in manufacture, for example, because of variations in density or porosity of the material.
Scintillator materials are chosen for high effective atomic number Zeff, light output that is a good spectral match to the detector, and a reasonable light yield, preferably >20 photon/keV. Some scintillators have a higher light yield, e.g. 40 photon/keV, including doped halides such as CsI, and gadolinium oxy-sulphide (GOS). Other scintillators, such as lead tungstate, and scintillating fibre-optics, have a higher x-ray stopping power, but a lower light output, e.g. <10 photons per keV. However, matching of a scintillator to an imaging device has tended to be only by experimentation. Moreover, there is often excess signal available from the scintillator.
Use of a range of typical scintillators of different thicknesses, for example to facilitate low-sensitivity, high-resolution and high-sensitivity, low-resolution options, and typical manufacturing variations of particular scintillators, can easily result in an order of magnitude variation in per pixel signal yield per x-ray photon.
In a photoconductor-based imager an imager is coated with, or attached to, a photoconductor layer. This layer has a bias field Vbias applied, for example by means of opposed metal contacts. When charge is generated in the photoconductor, the bias field sweeps the generated charge into a pixel structure. X-ray sensors based on a photoconductor are well known. Mammography and chest radiography applications typically use such sensors. Typical photoconductor materials include amorphous selenium, PbO, PbI2, HgI2 and CdZnTe.
The signal yield is often very high, e.g. 200e− (signal)/keV (photon), i.e. a single 30 keV x-ray photon will generate 6000 signal electrons. In an integrating imaging mode, if applied to a 20 μm pixel, as few as 50 x-ray photons may cause pixel saturation. In the current state-of-the-art, pixels are typically 50 μm or larger, in order to provide sufficient charge storage capacity. However, this imposes a limit on the spatial resolution of the imager.
In an intrinsically sensitive imager signal is directly generated in the imager pixel. X-ray sensors with intrinsic sensitivity are well known. Industrial and security and other scientific applications typically use such sensors. Typical imager materials include cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe), and also intrinsic silicon detectors, e.g. of 300 micrometer thickness.
The signal yield is often very high, e.g. 276e− (signal)/keV (photon), i.e. a single 30 keV x-ray photon will generate 8280 signal electrons. In an integrating imaging mode, if applied to a 20 μm pixel, as few as 36 x-ray photons may cause pixel saturation. In the current state-of-the-art, pixels are typically 50 μm or larger, in order to have sufficient charge storage capacity. However, this imposes a limit on the spatial resolution of the imager.
JP-11352230-A discloses an x-ray image detector having an electrochromic film sandwiched between a scintillator and a solid-state detector. Optical transmittance of the electrochromic film is adjusted by application of an electric field variably to attenuate light from the scintillator to the solid-state detector. This requires a relatively complicated structure compared with a standard solid-state imaging sensor without variable attenuation. The spatial resolution of the sensor may be degraded by a diffusing effect of the electrochromic layer. The electrochromic layer may add extra defects to the image.
Where imaging devices are used with fixed iris lenses and there is a need to control exposure, this may be done by varying a sensor integration time: a method often called electronic shuttering. Electronic shuttering is more precise than mechanical shuttering and can include times shorter than possible with a mechanical shutter. Integration time is decreased, e.g. to 10 μs and less, for imaging relatively bright objects to avoid saturating the pixels, or is increased, e.g. to 20 ms and more, to increase grey scale resolution of relatively dim objects. In video imaging applications the integration time is less than an inverse of the frame rate.
U.S. Pat. No. 6,825,059 discloses a controllable electronic shutter included in each pixel cell in order to provide a more efficient and versatile way of setting integration time of the array or part of the array. In some applications—e.g. fast shutter, motion capture or snap shot imaging, it is advantageous for all the pixels to be integrated simultaneously for the same period of time. In each pixel, signal charge is dumped or read.
In all cases, it is implicit that there will only be one on/off dump cycle per image or video frame. The rate of arrival of photons is relatively high. When signal charge is collected, all the signal charge is retained, but the signal is only collected from some of the incident photons. The yield of signal charge is fixed at the intrinsic rate of one signal electron from each detected photon Given the high number of photons per frame, e.g. of the order of 100,000, the photon shot noise limit is not usually a problem
There is also disclosed coating of an imager with a scintillator, which accepts x-ray radiation and then emits green light to the imager. However, in all cases, it is implicit that there will only be one on/off dump cycle per image or video frame. When signal charge is collected, all the signal charge is retained, but the signal is only collected from some of the incident photons.
U.S. Pat. No. 6,097,021 discloses an optical sensor array with a switch control logic circuit to control start and stop times of integration periods of each optical sensor, in particular for document scanning to avoid skew images. There is no disclosure of the use of such control logic in other than an optical sensor.
U.S. Pat. No. 6,403,965 describes an x-ray imager wherein the pixel signal per incident x-ray photon may be increased via amplification in a high electric field in an x-ray-to-charge converting layer. The technique is said to be of benefit at very low signal levels. However, the yield of signal per incident photon, already high, is increased and so the pixel has to be even larger, approx 200 μm, to provide very high well capacity but providing low resolution. The signal to noise is still photon shot noise limited at high flux levels. There is a possibility of avalanche breakdown in the high electric field (1 E8 V/m).
It is an object of the present invention at least to ameliorate the aforesaid shortcomings in the prior art.